High-Temperature Proton-Exchange Membrane Fuel Cells (HT-PEMFCs) are a candidate for electrical energy supply devices in more and more applications. Most notably in the aeronautic industry. Before any use, an HT-PEMFC is preheated and after that supplied with its active gases. Only at this state, the diagnostics can be performed. A method of testing not requiring a complete start-up would be beneficial for many reasons. This article describes an extended version of the charging and discharging diagnostic method of HT-PEMFCs with no active gases. This extended approach is named “Test Without Active Gases” (TWAG). This paper presents original research on the influence of nitrogen temperature and pressure on the HT-PEMFC response to charging and discharging. A lumped-element model of an HT-PEMFC is also presented. A numerical result of using this model to recreate an experimentally obtained curve is also presented.